The molecular cloning of restriction fragments isolated from mouse DNA that contain sequences that hybridize to an ecotropic MuLV specific probe is in progress. Preliminary analysis by Southern blot hybridization determined the size by preparative agarose gel electophoresis. The fragments containing ecotropic proviral sequences were isolated and are being cloned in cosmid vectors. These clones will be used in studies to determine if the basis for the difference in ecotropic MuLV expression in AKR and BALB/c mice results from differences in the proviral sequences or in the flanking cellular sequences. A similar approach will be employed in the isolation and molecular cloning of unique DNA restriction fragments that have been observed in AKR/J thymoma DNAs. The rule of new MuLV inserts the generation of thymoma will be examined by DNA and RNA blot hybridization studies.